I. Peripheral Arterial Disease
Peripheral Arterial Disease (PAD) is a progressive disease. In this disease, lesions of the arteries are formed by accumulation of plaque and neointimal hyperplasia causing an obstruction of blood flow. Plaque (the build-up of cholesterol, cells, and other fatty substances) is often friable and may dislodge naturally or during an endovascular procedure, possibly leading to embolization of a downstream vessel.
It is estimated that 12 million people in the United States suffer from PAD that if left untreated has a mortality rate of 30 percent at five years. There are approximately 160,000 amputations each year from critical limb ischemia, the most severe subset of patients having PAD. The prevalence of PAD is on the rise, with risk factors including age, obesity, and diabetes.
Endovascular clearing procedures to reduce or remove the obstructions to restore luminal diameter and allow for increased blood flow to normal levels are known. Removing the plaque has the effect of removing diseased tissue and helps to reverse the disease. Maintaining luminal diameter for a period of time (several to many weeks) allows remodeling of the vessel from the previous pathological state to a more normal state. It is also the goal of an endovascular clearing procedure to prevent short term complications such as embolization or perforation of the vessel wall, and long term complications such as ischemia from thrombosis or restenosis.
II. Prior Treatment Modalities
Unlike coronary artery disease, current treatment options for PAD, including PAD in the arteries of the leg, have significant limitations for at least three main reasons: A) large volumes of plaque build up in very long, diffuse lesions, B) low blood flow promotes thrombus formation and plaque buildup, and C) arteries of the leg are bent, twisted, stretched, and pinched shut during routine movement.
Various treatment modalities have been tried to accomplish treatment goals. In atherectomy, plaque is cut away, or excised. Various configurations have been used including a rotating cylindrical shaver or a fluted cutter. The devices may include some form of shielding by a housing for safety purposes. The devices may incorporate removal of debris via trapping the debris in the catheter, in a downstream filter, or aspirating the debris, for example. In some cases a burr may be used instead of a cutter, particularly to grind heavily calcified lesions into very small particle sizes. Aspiration may also be used with a burr-type atherectomy device.
A current example of an atherectomy device is the SilverHawk® Plaque Excision System by Fox Hollow Technologies. The SilverHawk has a number of limitations including the length of time the procedure takes to clear a lumen, it requires multiple devices and repeated catheter exchanges, it produces embolic debris, and it uses an unguarded cutter design that requires great patience and care to open the lumen while sparing the vessel wall. In use, the physician advances the catheter through the lesion, shaving plaque off of the artery walls and collecting the plaque in a long receptacle (nosecone) at the tip of the catheter (which must have enough room in the vessel to pivot to access the cutting blade). As the receptacle fills, the catheter must be removed, the receptacle emptied, and the procedure repeated until enough plaque is removed to restore normal blood flow. The procedure may include replacing the catheter with a larger diameter catheter to expand the size of the clearing. The long receptacle at the tip of the catheter limits the use of the device to mainly straight lumens.
Balloon angioplasty is another type of endovascular procedure. Balloon angioplasty expands and opens the artery by both displacing the plaque and compressing it by expanding a balloon in the artery, with some variations including a drug coated balloon. Balloon angioplasty is known to cause barotrauma to the vessel from the high pressures required to compress the plaque, and can also cause dissection of the vessel wall. This trauma leads to an unacceptably high rate of restenosis. Furthermore, this procedure may not be efficient for treatment of elastic-type plaque tissue, where such tissue can spring back to occlude the lumen.
Cryoplasty has been available for only a few years and has provided only limited positive results. With cryoplasty, the main problem appears to be restenosis after an extended period, such as a year. The technique is similar to balloon angioplasty procedures used in heart vessels, except stents are not used to keep the blood vessel open. With cryoplasty, the balloon is cooled to about −10 degrees Celsius (14 degrees Fahrenheit) by evaporating liquid nitrous oxide into a gas upon entering the balloon. The plaque clogging the artery cracks when it freezes, allowing for a more uniform dilation of the blood vessel than occurs in a standard angioplasty procedure.
Various forms of laser atherectomy have been developed and have had mixed results. One main limitation of a laser system is that the laser can only be effectively used in a straight lumen, and is less effective in or around tortuous lumens. When the laser is in position, it emits pulsating beams of light that vaporize the plaque. Laser systems have been less effective for removing calcified legions because of the laser properties.
Stenting may also be used as a treatment option. On their own, stents, including drug eluding stents, fail to perform well in the peripheral vasculature for a variety of reasons. A stent with the necessary structural integrity to supply sufficient radial force to reopen the artery often does not perform well in the harsh mechanical environment of the peripheral vasculature. For example, the peripheral vasculature encounters a significant amount of compression, torsion, extension, and bending. Such an environment may lead to stent failure (strut fracture, stent crushing, etc.) that eventually compromises the ability of the stent to maintain lumen diameter over the long-term. Stenting is also susceptible to in-stent restenosis, typically at a restenosis rate of 30 percent or higher. Stent fracture or restenosis may require subsequent vascular bypass surgery, which is invasive and is limited in the types of lesions or artery obstructions that may produce acceptable results. Stenting is not advisable in regions which would be candidates for proximal or distal anastamosis during surgical bypass procedures, because a stent in that region makes bypass difficult or impossible.
On the other hand, a stent that is able to withstand the harsh mechanical aspects of the periphery often will not supply enough radial force to open the vessel satisfactorily. In many cases, medical practitioners desire the ability to combine endovascular clearing procedures with stenting. Such stenting may occur prior to, after, or both before and after the endovascular clearing procedure.
Accordingly, a need remains for devices, systems, and methods that allow for improved atherectomy systems that are able to navigate through tortuous anatomy and clear materials from body lumens (such as blood vessels) where the systems includes features to allow for a safe, efficient and controlled fashion of shaving or grinding material within the body lumen while minimizing procedure times. In addition, there remains a need for systems that allow steering of the distal portion of the system while navigating through tortuous anatomy. The ability to steer assists the physician in accessing tortuous anatomy and can further assist in delivering a guidewire into the entrance of angled or tortuous vessel bifurcation/segments. This is possible because variations of the steerable atherectomy catheter system described herein can also function as a ‘shuttle catheter’, where the physician can aim the distal tip into the vessel to be accessed and advancing the guidewire into that vessel from within the catheter
There also remains a need for devices that are configured to steer but will remain in a straight configuration when not being articulated. It is generally known that conventional catheters that take a shape often bias to one side either through repeated articulation or even after being left in packing for any given period of time. Accordingly, when such steering features are combined with tissue debulking systems, there remains a risk of injury if the tissue debulking system has an undesirable bend when the device is intended to be in a straight configuration.
The debulking devices, systems, and methods described herein address the problems noted above as well as provide significant improved features to allow a physician to steer a debulking device through tortuous anatomy and remove tissue at a target site.